1. Doppler-free two-photon absorption spectroscopy of vibronic excited states of naphthalene assisted by an optical frequency comb
UNIV. of Electro-Communications
AFukuoka UNIV.
BKyoto UNIV.
Akiko NISHIYAMA
Kazuki NAKASHIMAA
Masatoshi MISONOA
Masaaki BABAB

2. Introduction ～ High-Resolution Spectroscopy
Singlet state
Triplet state T1 T2 T3
●Doppler-free spectroscopic technique
→ Rotationally resolved spectra
　→ Detailed structures of excited states S0 S1 S2 S3
●Precise spectroscopic system
　→ Detection of small effect
　… Frequency shift, Line broadening,
Zeeman effect, …
　→ Dynamics in excited states
~ 1015 Hz
●Requirement for the system
accuracy and resolution
～ = 1 MHz

3. Target ～ Excited States of Naphthalene
<Two-Photon Transitions of Naphthalene>
n4(b1u)+n7(ag) x z
2545 cm-1
Lemon
n4(b1u)+n8(ag)
2260 cm-1
Yellow
n4(b1u)
1560 cm-1
Orange
S11B1u
Wavelength [nm]
S01Ag

4. Motivation ～ Study of Line Broadening Effect due to IVR
<Fluorescence spectrum>
Excess energy > 2000 cm-1
Intramolecular Vibrational energy Redistribution (IVR) becomes dominant interaction
Excess energy
1560 cm-1
Orange
In this study,
We observe beginning of the IVR phenomena precisely.
2260 cm-1
Yellow
2545 cm-1
Lemon
S. M. Beck, et al., J. Chem. Phys. 74, 43 (1981).

5. Experimental Setup linewidth of 100 kHz scan speed of 2 MHz/sec
free-running PD
Naphthalene Cell
CW dye Laser
PBS
Photomultiplier
Tube I2
λ/4
AOFS
APD
RF Synthesizer
Ti:S Comb
Computer
Photonic
Crystal Fiber
Diffraction
Grating
Frequency
Counter
stabilized to GPS disciplined clock
AOFS: Acousto-Optic Frequency Shifter
EOM: Electro-Optic Modulator
APD: Avalanche Photodiode
PBS: Polarizing Beam Splitter

6. Doppler-free two-photon absorption spectroscopy
Mirror
Mirror
hf(1 + u/c) u
2hf f
f(1 - u/c)
f(1 + u/c)
hf(1 - u/c)
- Doppler free
- Enhancement of light intensity for excitation
- Selection rule is different from one photon absorption
- Transition in UV region is excited by a visible laser
- All molecules contribute to the signal independent of their velocities

7. Experimental Setup linewidth of 100 KHz scan speed of 2MHz/sec
free-running PD
Naphthalene Cell
CW dye Laser
PBS
Photomultiplier
Tube I2
λ/4
AOFS
APD
RF Synthesizer
Ti:S Comb
Computer
Photonic
Crystal Fiber
Diffraction
Grating
Frequency
Counter
stabilized to GPS disciplined clock
AOFS: Acousto-Optic Frequency Shifter
EOM: Electro-Optic Modulator
APD: Avalanche Photodiode
PBS: Polarizing Beam Splitter

8. Optical Frequency Comb
Cs atomic clock
on GPS satellite
fbeat
< 10-11
Frequency
fCEO
frep fn
flaser
flaser = nfrep + fCEO + fbeat

9. Experimental Setup linewidth of 100 kHz scan speed of 2 MHz/sec
free-running PD
Naphthalene Cell
CW dye Laser
PBS
Photomultiplier
Tube I2
λ/4
AOFS
APD
RF Synthesizer
Ti:S Comb
Computer
Photonic
Crystal Fiber
Diffraction
Grating
Frequency
Counter
stabilized to GPS disciplined clock
AOFS: Acousto-Optic Frequency Shifter
EOM: Electro-Optic Modulator
APD: Avalanche Photodiode
PBS: Polarizing Beam Splitter

10. Frequency Measurement of Scanning Laser
fdye
(a)
CW dye
frep
(a)
frep
PBS
time
λ/4
fAO
(b)
(c)
AOFS
(b)
frep
Ti:S comb
time
(d)
fdye +
fAOM
(c)
frep
distribution
fbeat
fbeat [MHz]
frep
frep
time
fbeat
(d)
time
Accuracy ～ 10s kHz
A. Nishiyama, et al., Opt. Lett. 39, 4923 (2014).

11. Measurement of Two-Photon Spectrum of Naphthalene
fAO
[MHz]
1080
1000
41.2
fbeat
[MHz]
40.6
1560 cm-1
Naphthalene
two-photon
absorption
[arb. unit]
Orange
Wavenumber [cm-1]
A. Nishiyama, et al., J. Mol. Spectrosc. 318, 40 (2015).

12. Target ～ Excited States of Naphthalene
<Two-Photon Transitions of Naphthalene>
n4(b1u)+n7(ag) x z
2545 cm-1
Lemon
n4(b1u)+n8(ag)
2260 cm-1
Yellow
n4(b1u)
1560 cm-1
Orange
S11B1u
Wavelength [nm]
S01Ag

13. Observed Naphthalene Spectra
Lemon
origin
n4(b1u)+n7(ag)
Excess energy
= 2545 cm-1
Yellow
origin
n4(b1u)+n8(ag)
Excess energy
= 2260 cm-1
Orange
origin
n4(b1u)
Excess energy
= 1560 cm-1
Observed rovibronic lines are assigned as Q(Ka)Q(J) transitions

14. Observed rovibronic spectra ～ Q(Ka)Q(J) transitions
J=26, Ka=0,1, Kc=26
J=25, Ka=1,2, Kc=24
J=24, Ka=2,3, Kc=23
Orange
n4(b1u)
Yellow
n4(b1u)+n8(ag)
Lemon
n4(b1u)+n7(ag)

15. Molecular constants Tentative value S1B1u [cm-1] Orange band n4(b1u)
Yellow band
n4(b1u)+n8(ag)
Lemon band
n4(b1u)+n7(ag) A
(3)
(16)
(48) B
(2)
(25)
(58) C
(2)
(16)
(52) Dj
1.35 (10) ×10-9
4.1 (10) ×10-7
-1.9 (8) ×10-7
Djk
-1.26 (5) ×10-8
-2.0 (3) ×10-6
2.0 (9) ×10-7 Dk
1.52 (4) ×10-8
1.6 (2) ×10-6
-8.8 (12) ×10-8 dj
3.51 (8) ×10-9
2.1 (5) ×10-7
-9 (4) ×10-8 dk
1.03 (1) ×10-7
-7.2 (9) ×10-7
-4 (5) ×10-8
Origin
RMS
line
963
287
121
S0Ag [cm-1]
Ground state
n= O. Pirali, et al., Phys. Chem. Chem. Phys. 15, (2013).
(124)
(37)
(140)
Dj×109
0.528 (49)
Djk×108
(145)
Dk×108
(112)
dj×109
0.1752
dk×107

16. Interaction between excited states in Orange band
＜Frequency shifts＞
Ka + Kc = J
＜Change of linewidths＞
Signal intensity
[arb. unit]
Wavenumber [cm-1]
2.48 [MHz]
Ka + Kc = J+1
[MHz] J 30 10 20
Ka = 0
Ka = 1
Ka = 2
Ka = 3
Ka = 4
200
-200
Ka = 5
Ka = 5 J 30 10 20
[MHz] 2 1 3
Linewidths have
no J value dependence
Coriolis interaction 16

17. Linewidth of the Ka = 0 lines
Averaged linewidth
(Ka=0 series)
Natural width
(calculated from the life time※)
2.71 MHz
0.91 MHz
Orange　n4(b1u)
3.37 MHz
1.71 MHz
Yellow n4(b1u)+n8(ag)
Lemon n4(b1u)+n8(ag)
3.51 MHz
1.63 MHz
※U. Boesl et. al. Chem. Phys. Lett, 42, 16 (1976).
・Transit-time broadening = 0.64 MHz
・Pressure broadening = 0.53 MHz (20 Pa)
・Laser linewidth×2 ＝ 0.26 MHz
・Natural width
1.43 MHz
<Contribution to the spectral linewidth>
Variation of life time was observed in rovibrational linewidths

18. Summary
To analyze the intramolecular vibrational energy redistribution (IVR) in the excited state of naphthalene, we observed three two-photon transitions with excess energies (Ex) about 2000 cm-1,
S11B1u n4(b1u) + n7(ag) ← S01Ag Ex = 2545 cm-1
S11B1u n4(b1u) + n8(ag) ← S01Ag Ex = 2260 cm-1
S11B1u n4(b1u) ← S01Ag Ex = 1560 cm-1
We analyzed rotational dependences of the linewidth, and observed variation of the life times in rovibronic linewidth.
Analysis of Ka dependences of the linewidth is in progress.